The proposed study will explore the pathogenesis of human obesity and overweight from a new angle - the possible effects of suspected industrial chemicals in a prospective study. While over-nutrition and sedentary lifestyle are well-established risk factors of obesity, a role of other potential obesity-contributing factors is highy likely and needs to be elucidated. Environmental chemicals known as obesogens may upset body weight regulation. Perfluorinated compounds (PFCs) are ubiquitous, persistent chemicals that interfere with energy homeostasis in animal models. However, because of species-specific toxicokinetics, tissue distribution, and mechanisms of action of PFCs, the relevance of animal data to humans is uncertain. Human evidence originated primarily from cross-sectional studies is largely consistent with the hypothesis that PFCs are associated with higher body weight, but lack of prospective data precludes any causal inference. The investigators will, therefore, prospectively examine the roles of these pollutants in relation to human weight regulation in the well-designed and rigorously-implemented POUNDS LOST Trial that was recently completed. The participants of this 2-year trial consist of 811 men and women who were randomized to one of four calorie- reduced diets at baseline. While the diets had similar effects on weight change, substantial between-individual variability was observed in short-term weight-loss and subsequent weight-regain that cannot be explained by compliance or established obesity risk factors. The investigators will analyze banked plasma for PFCs to examine whether baseline PFC levels are associated with less weight-loss and greater weight-regain among ~700 participants who completed 6-month and 2-year follow-up. In addition, we will examine associations with waist circumference and fat content of whole body, abdomen, and liver that were longitudinally recorded in the trial. The investigators also aim to elucidate possible mechanisms by evaluating plasma PFC concentrations in regard to gene expression profiles in adipose tissue, as well as changes of adipokines and thyroid hormones. The multidisciplinary research team is comprised of experts with qualifications and experience in obesity research, environmental epidemiology, nutrition, genetics, and biostatistics, thus ensuring successful implementation of the current project. The prospective evidence will be directly relevant to humans, will likely lead to new insight and hypotheses regarding obesity and related disease outcomes, and will facilitate making more comprehensive and effective strategies for obesity prevention and treatment.